Printed circuit boards are currently used in practically every type of electronic equipment. A typical printed circuit board (PCB) consists of multiple layers of composites of organic and inorganic materials, with internal and external wiring, allowing electrical components to be mechanically supported and electrically connected. The growing complexity of the electronic circuitry imposed the requirement of allowing greater numbers of components to be installed on a PCB of a given size. The PCB manufacturing technology trend has thus been towards increasing the number of interconnected layers, greater pass-through hole densities, and finer line (trace) widths. The resulting structural compactness provides significant advantages but in addition has created a number of problems.
One such problem is to provide power to the individual integrated circuits mounted on the PCB. In a typical prior art multi-layered PCB, power is supplied to the externally mounted components via a distribution core consisting of a fiberglass/epoxy dielectric element sandwiched between a copper foil power plane and a similar ground plane. Other materials from which the dielectric element is commonly made are Teflon and Polyamide, used to alter the dielectric constant for better signal propagation characteristics. The dielectric constant .epsilon..sub.r of the standard FR-4 fiberglass/epoxy dielectric typically ranges from approximately 4.0 to 5.5 and could be as low as 2 for alternate materials. Such low dielectric constants provide a capacitance of approximately 10 picofarads per square inch or less, which is not sufficient to satisfy the inrush current requirements of a standard integrated circuit.
As a result, typical PCB assemblies require the use of additional, externally mounted decoupling capacitors. Such capacitors, however, occupy considerable PCB surface space, require extra assembly (insertion) time, and increase the overall cost of a completed PCB unit. In addition, each capacitor also constitutes a potential point of failure that reduces the inherent system reliability.
Some of the above-mentioned problems have been addressed in the past. For example, U.S. Pat. No. 4,511,951 to Gottlieb, U.S. Pat. No. 4,532,572 to Schilling, U.S. Pat. No. 4,622,619 to Schilling et al., U.S. Pat. No. 4,630,170 to Kask, and U.S. Pat. No. 4,726,777 to Billman propose improved decoupling capacitors for printed circuit boards. In each case, the decoupling capacitors are built as discrete components, the improvement being in the manufacturing process, in the control of design parameters and the positioning and/or ease of insertion into the board.
Throughout the evolution in PCB technology, however, little progress has been made with respect to building capacitors and other electronic devices as integrated elements during the multilayer PCB manufacturing process. The lack of progress is primarily due to technological problems associated with such integration. It is difficult to integrate components into PCB substrates due to the fact that PCB manufacturing processes and circuit manufacturing processes are basically incompatible in their required cleanliness, thermal cycle, photolithography and other requirements. Thus, most prior art solutions fail to address the problem of eliminating the need for separate components, which reduce the available board space, require separate assembly, reduce inherent reliability and increase the overall cost of the product.
U.S. Pat. No. 5,162,997, which is incorporated herein by reference, makes significant progress in the solution of this problem by integrating a single element high capacitance power distribution core in the PCB manufacturing process. The power distribution core is interconnected with components mounted on either side of the board using standard PCB assembly technology. The high capacitance core consists of a signal ground plane and a power plane separated by a dielectric core element having a high dielectric constant .epsilon..sub.r. Using a glass fiber and ferro-electric nano-powder loaded epoxy construction, the power distribution core exhibits a capacitance of approximately 0.1 microfarads per square inch. As a result, a PCB with such high capacitance power distribution core typically requires no additional decoupling for the associated integrated circuits, and thus obviates the need for externally mounted decoupling capacitors.
Notwithstanding the advantages of using this high capacitance power distribution core, it is perceived that the approach has not reached its full potential. In particular, it is often desirable to create a better match between the dielectric, mechanical, thermal expansion and other properties of the materials used in the power distribution core and the remainder of the board. The use of different types of integrated circuits, such as TTL and ECL, on the same board in addition makes it desirable to provide separate areas of variable decoupling capacitance to meet different voltage and/or polarity power supply requirements. Using a single element as a high capacitance power distribution core significantly reduces the designer's flexibility in this respect.